Isolation, identification, virulence potential and genomic features of Tenacibaculum piscium isolates recovered from Chilean salmonids.

Tenacibaculum piscium, a gram-negative bacterium isolated from the skin ulcers of sea-farmed fish, has only been described in Norway. In the present study, we examined 16 Chilean Tenacibaculum isolates recovered from different organs in moribund and dead Atlantic salmon (Salmo salar), Rainbow trout (Oncorhynchus mykiss) and Coho salmon (Oncorhynchus kisutch) cultured at different fish farms between 2014 and 2018. The present study applied biochemical, phenotypic, fatty acid and whole-genome sequence-based analyses to confirm the taxonomic status of the Chilean isolates. The obtained results are the first to confirm the presence of T. piscium in Chile and in Coho salmon, thus extending the recognized geographical and species distribution of this bacterium. Subsequent bath-challenge assays in Atlantic salmon utilizing three T. piscium isolates obtained from different hosts resulted in low cumulative mortality (i.e. 0-35%), even after exposure to an unnaturally high concentration of bacterial cells (i.e. > 107 cells/ml). However, scale loss and frayed fins were observed in dead fish. In silico whole-genome analysis detected various genes associated with iron acquisition, encoding of the type IX secretion system and cargo proteins, resistance to tetracycline and fluoroquinolones and stress responses. These data represent an important milestone towards a better understanding on the genomic repertoire of T. piscium.

Serological diversity in Flavobacterium psychrophilum: A critical update using isolates retrieved from Chilean salmon farms.

Chile is currently the second largest producer of farmed salmon worldwide, but Flavobacterium psychrophilum, as one of the most detrimental pathogens, is responsible for major losses during the freshwater culturing step in salmonid fish farms. An antigenic study conducted 10 years ago reported four serological groups using 20 F. psychrophilum Chilean strains. To reduce disease outbreaks and to develop vaccine candidates, antigenic knowledge needs to be regularly updated using a significant number of additional recent F. psychrophilum isolates. The present study aimed at investigating the serological diversity of 118 F. psychrophilum isolates collected between 2006 and 2018 from farmed Atlantic salmon (Salmo salar), rainbow trout (Oncorhynchus mykiss) and coho salmon (Oncorhynchus kisutch). The current study supports an expansion of the known antigenic groups in Chile from 4 to 14. However, the use of the slide-agglutination technique for serotyping is costly, is labour-intensive and requires significant technical expertise. Addressing these points, the mPCR-based procedure was a very useful tool for serotyping the collected Chilean F. psychrophilum isolates. This technique revealed the presence of diverse mPCR serotypes (i.e. types 0, 1, 2 and 4). Therefore, mPCR should be employed to select the bacterial strain(s) for vaccine development and to conduct follow-up, selective breeding or epidemiological surveillance in Chilean fish farms. Given the presented findings, changes to Chilean fish-farming practices are vital for ensuring the continued productivity and well-being of farmed salmonids.

Introduction, expansion and coexistence of epidemic Flavobacterium psychrophilum lineages in Chilean fish farms.

Chile is one of the countries where the development of salmonid farming has been the most successful. The first importation of salmonids in Chile from the northern hemisphere dates back to the late 19th century and the country now ranks as the world second largest producer of farmed salmon. However, the fast increase of infections caused by the bacterium Flavobacterium psychrophilum is a growing concern for this local industry. This pathogen, also recognized as an important problem worldwide, has been first reported in Chile in 1993 and is currently affecting all three cultivated salmonid species: Atlantic salmon (Salmo salar), Coho salmon (Oncorhynchus kisutch) and rainbow trout (O. mykiss). Here we conducted a MLST (multi-locus sequence typing) analysis of the local genetic diversity of F. psychrophilum to better understand its origin and propagation in the country, and to suggest practices that could contribute to its control in the future. A total of 94 bacterial isolates, collected from the main production zones, were analyzed and compared to those of other origins already available. The data reveal the country-wide distribution of several genotypes closely related to those that are the most prevalent in European and North American fish farms, and overlapping host fish species of the different lineages. This population structure is probably the direct consequence of local fish farming practices that relied until recently on massive import of fish eggs (e.g., 78 million of eggs in 2012) and where mixed-species farms and fish transportation across the country are common.